The main objective of the work proposed here is to examine the relationship between DNA sequence organization and chromatin structure in the sea urchin. It is felt that a definable structural relationship exists and that knowledge of this relationship will yield new information on control of gene expression at the level of transcription. This will be approached in three major steps: 1. Purification of DNA sequences which are resistant to light DNase I digestion in nuclei. 2. Isolation of various DNA sequence components. a. Total non-repeated DNA sequences by kinetic fractionation. b. Total long and short repeated DNA sequences by kinetic fractionation and gel chromatography. c. Cloning of sea urchin DNA fragments containing both repeated and nonrepeated sequences in E. coli. 3. Reassociation of the various DNA sequence components with the DNase I resistant sequences.